The invention relates generally to a packing ring for use with ribbed conduits. More specifically, the invention pertains to a packing ring of elastic material.
A ribbed conduit or tube in the context of this invention is a tube having circumferentially extending radially protruding stiffening ribs on its outer cylindrical surface in an axial equidistant sequence. For economy of both material and weight, the stiffening ribs are not solid but hollow. Depending on the minimum mechanical strength required, the ribs have thin walls. The stiffening ribs may open radially to the inside of the conduit so as to themselves form the inner circumferential surface of the tube. For example, this is the configuration of a so-called corrugated tube. In addition, the stiffening ribs may be disposed on the outside on the circumferential surface of a cylindrical tube having smooth internal walls, as in the narrow sense of ribbed tubing. Ribbed tubing of this type is made predominantly of a synthetic plastic or of metal, but other materials may also be used. Ribbed plastic tubes are used primarily in the construction industry for underground drainage pipes.
Ribbed tubing of the type discussed above is usually manufactured without integral union sockets or connecting flanges. Adjacent individual tube sections are ordinarily joined together by means of a push-on sleeve coupling having a smooth cylindrical inner surface. The sleeve coupling concentrically surrounds and overlaps pointed ends of the ribbed tubes which have front surfaces in abutment with each other. Such a connecting sleeve may be made of a synthetic plastic, metal or, in principle, any other material, including for example a ceramic.
Increasingly high sealing requirements are being specified for such tube connectors, and particularly for ribbed tubing. This is especially true for water carrying tubing used in underground installations such as in the field of sewage disposal. In accordance with the current state of the art, the sealing between the front and of a ribbed tube with a smooth inner wall of the sleeve of a tube-sleeve connection is effected by means of an annular gasket with round cross-section, a so-called O-ring. The O-ring is inserted into one groove between two adjacent ribs at the front end of the tube section. The expression "groove" in the context of the invention is thus the valley or annular depression formed between two immediately adjacent circumferential stiffening ribs of the ribbed tube, which ribs protrude radially in the outward direction.
Particularly in the case of a ribbed tube made of a plastic material, the axial cross section of a groove is essentially trapezoidally-shaped with the large base of the trapezoid corresponding to the radially outward, open side of the groove. The cross-sectional diameter of the O-ring gasket customarily used as the packing for sleeve connections is slightly smaller than the average axial width between the upper edges of the groove. The annular diameter of the O-ring gasket is typically chosen so that just over half the diameter of the gasket is drawn into the groove with a slight prestressing in the radial direction.
During the application of the sleeve while assembling a joint, the protruding part of the O-ring gasket is pressed into the groove. In the process, the major portion of the pressure generated by compressing the gasket acts essentially in the axial direction on the two flanks or side walls of the groove. The sealing of this sleeve connection thus occurs essentially between the inner wall of the sleeve and the uppermost portion of the O-ring gasket, as well as between the lateral surfaces of the O-ring gasket and the side wall surfaces of the groove, i.e., the ribs. Additional radial pressure between the upper surface of the O-ring gasket and the inner wall of the sleeve will cause only a minor increase in sealing contact between O-ring gasket and the bottom or root portion of the groove.
A disadvantage of this type of packing is the high level of axial pressure acting on the side walls of the groove or the ribs. The lateral walls of ribs are designed for radial stiffening and are not, as a rule, stiffened against an axial force in most ribbed tubes having typical hollow ribs. The stiffening ribs are thus forced apart, or deflected, by the lateral pressure of the O-ring gasket. This axial deflection may occur rapidly or gradually after extensive exposure to the lateral pressure. In all cases the latter (axial) yielding of the groove side walls leads to the reduction of the axially acting sealing forces. Specifically, due to the plastic deformation of the groove side walls and the axial deformation of the O-ring gasket caused by elimination of the lateral resistance to deformation, the already slight radial pressure component, or the equivalent radial sealing force, is decreased. As the result of the deformation forces generated by the internal pressure of the O-ring gasket acting axially on the groove sidewalls and the subsequent plastic deformation of the stiffening ribs, an appreciable reduction occurs of the limiting pressure which the sleeve connection can withstand. Conventional O-ring gaskets thus are frequently incapable of satisfying pressure tightness requirements specified in various test standards.
An initial solution of the problem is to simply increase the original cross-sectional diameter of the O-ring gasket to accommodate the internal pressure loss of the gasket caused by the plastic deformation of the ribs. But, this solution must be rejected. An O-ring gasket strengthened in this manner would hardly permit the sliding of the sleeve over the front end of the tube for two reasons: first, the force required to place the sleeve onto the tube would be excessive, and second, the O-ring gasket, which obviously would project higher above the groove, would roll out of the groove during assembly of the sliding ring and would slide off the rib surface at the tube front end. This would result in uncontrolled twisting of the O-ring gasket and altogether uncontrollable sealing conditions, both of which are intolerable in underground sewage line installations.